1. Field of the Invention
The present invention relates to a fixing apparatus included in an image forming apparatus such as an electrophotographic copying machine or an electrophotographic printer.
2. Description of the Related Art
In recent years, an image forming apparatus such as a copying machine or a printer has been requested to achieve shortening of first printout time (FPOT) and more energy saving. Under these circumstances, as a fixing apparatus included in the image forming apparatus, a fixing apparatus using a cylindrical film of a low heat capacity is widely used. The fixing apparatus typically includes a film, a heater that contacts an inner surface of the film, and a pressing member forming a nip portion with the heater via the film, heats a recording material having a toner image formed thereon at the nip portion while conveying it, and fixes the toner image on the recording material. Since time necessary for heating the film of the low heat capacity is short, FPOT can be shortened. Since it is not necessary to supply any large power during standing-by, the fixing apparatus is advantageous to energy saving.
In such a fixing apparatus, when continuous fixing is performed on a small-size recording material, an excessive temperature rise of a region (hereinafter, referred to as non-sheet passing portion) through which no recording material passes, in other words, a non-sheet passing portion temperature rise, may occur.
To deal with this problem, Japanese Patent Application Laid-Open No. 11-260533 discusses a technology for suppressing the non-sheet passing portion temperature rise by arranging heat transfer members having high thermal conductivity on rear and front surfaces of a heating member. However, concerning the non-sheet passing portion temperature rise, a highest temperature position varies depending on the size of a recording material in a direction orthogonal to a conveying direction of the recording material. In a member having a sectional shape uniform in a longitudinal direction as in the case of the heat transfer member discussed in Japanese Patent Application Laid-Open No. 11-260533, a heat capacity of the entire heat transfer member must be set high in order to suppress the non-sheet passing portion temperature rise irrespective of the size of the recording material. Thus, further shortening of FPOT is difficult.